Digilent 410-237P User Manual
Page 2
PmodGPS Reference Manual
www.digilentinc.com
page 2 of 4
Figure 1:
1PPS Pin Timing Diagram
Figure 2:
3DF Pin output without a fix
The PmodGPS also comes equipped with a
coin cell retainer for a 12.5 mm coin cell
battery. Users can significantly reduce the
amount of time that it takes to acquire the first
positional fix by installing a 3V coin cell battery.
With the battery installed the module can also
perform a hot start or a warm start instead of
cold starting where users have to supply power
to the VCC.
Note: The ground square solder pad of the
coin cell retainer may develop an oxide build
up that will keep the battery from making a
good connection. Users should simply scuff up
the square solder pad inside J3 to remove any
buildup. Oxide buildup may also occur if there
has not been a battery in the retainer for a
while.
A cold start takes one or two minutes while
outside in good conditions, and can take
several minutes more if conditions are worse
or the module is indoors. A hot start takes
three to five seconds and a warm start varies
depending on how long the module has sat
unpowered. Users can hot start the PmodGPS
only if it has acquired a fix within approximately
the last two hours and with accessible backup
power (coin cell battery) connected to the coin
cell retainer. A warm start occurs when the
battery is connected but the module has been
without power for more than two hours.
Users may utilize an external antenna at their
discretion by installing a Linx Technologies Inc.
CONSMA 003.062 module on header J4. The
antenna can speed up acquisition of GPS
signal in some conditions, especially if the
antenna is outdoors and the module is indoors.
The PmodGPS uses sentences based on
National
Marine
Electronics
Association
(NMEA) protocols for data output. Each NMEA
message begins with a ($) dollar sign. The
next five characters are the talker ID and the
arrival alarm. The PmodGPS talker ID is
“GP”
and the arrival alarm is the specific sentence
output descriptor. Individual comma separated
data fields follow these five characters. After
the data fields there is an asterisk followed by
a checksum. Each sentence should end with
refer to tables two through six at the end of this
manual.
User may configure some of the PmodGPS
characteristics by writing command packets to
the module. However, these functions are
more advanced and not all of the command
packets are openly distributed. Users may
change the baud rate to 38.4kBd (minimum
baud for 10Hz data acquisition) by issuing the
following command over the UART:
“$PMTK251,38400*27
The “*27” corresponds to a checksum, if users
want to substitute a different baud rate, then
they must calculate a new checksum using
GlobalTop’s Checksum Tool. The command
for changing the data acquisition from 1Hz to
10Hz is:
“$PMTK226,3,30*4
The same checksum process for changing the
baud rate applies to this change too. Contact
GlobalTop for more information on the
individual command packets, their complete